You need to add a table for readings for a given grease pot. In that table you will have the PotID, Date, Time, Person. The table will store all the readings for all of the pots for many days.
Then I think you will need several forms with subforms in them. For each one-to-many relationship (in your case you have, one line has many pots as well as one pot has many readings) you will want to make a form that shows the many in a continuous form that is linked to a form with the one side. Getting specific...
Create a form that is linked to the Lines table. Allow the user to navigate through each line. Now create a separate form linked to the LineGreasePot table. In the Format properties for the form, change the Default View to Continuous. This allows you to see all of the GreasePots (like in table view).
Now go back to the Line form and add a subform. Choose an existing form, whatever you called the GreasePots continuos form, and when the wizard asks how you want to link the form with the subform, use the LineID.
What should happen, is ass the user navigates through the lines, he/she should see all of the pots assigned to that line.
Then do something similar for the Pots and the PotReadings. Pots would be your form and PotReadings would be your subform.